In carbureted spark-ignition engines, one means of increasing the thermal efficiency (and thus the work output) of the engine is to increase the compression ratio, defined as the ratio of the maximum volume between the piston and the head to the minimum volume between the piston and the head. A higher compression ratio increases the thermal efficiency of the engine. It is therefore desirable for high-performance engines to have the greatest compression ratio possible.
However, the compression ratio cannot be increased indefinitely. The higher compression ratio causes higher temperatures and pressures in the combustion chamber's fuel-air mixture upon compression, thereby increasing the possibility of rapid self-ignition of the fuel-air mixture at some unplanned point during the work cycle. Such an event is commonly referred to as "knock" or autoignition.
A high degree of autoignition is undesirable because it lowers the engine efficiency and it also increases the chances of engine failure. Engine efficiency drops when autoignition occurs because cylinder gas expansion occurs at a time when its work potential cannot be fully utilized. For maximum work output, ignition is best begun shortly before the piston reaches top dead center so that peak combustion chamber pressures occur shortly after the piston reaches top dead center. Peak pressures at this point allow the work generated by the expanding combustion gases to be utilized over the greatest length of piston travel. The rapid pressure increases associated with autoignition can cause peak pressures to occur before this point, perhaps when the piston has not yet reached top dead center, thereby causing work losses by opposing the piston motion. Further, autoignition promotes heat losses from the engine because the combustion gases vibrate and "scrub" the cylinder walls as the shock wave from autoignition travels through the cylinder. Autoignition can also cause premature engine failure due to the extremely high temperatures it causes in the cylinder, as well as from the damage it causes by stress from the shock waves.
Autoignition is frequently triggered in high compression ratio engines by the normal process of ignition. The spark plug (or plugs) ignites the compressed fuel-air mixture, and as the flame front begins to travel from the spark site through the chamber, the end gas--the fuel-air mixture in the cylinder farthest away from the spark plug(s)--is additionally compressed by the expansion of the gases behind the flame front. The mixture is already at a state of high temperature and pressure due to its previous compression, and if this additional compression of the end gas causes further temperature and pressure increases, the end gas may autoignite.
While autoignition may be decreased by using a fuel with a higher octane rating--and therefore a higher ignition temperature--it is also helpful to increase the speed of combustion, leaving little time for autoignition of the end gas to occur. A number of steps may be taken to increase the rate of combustion, such as the use of modified fuels or multiple spark plugs. Another method involves increasing the turbulence in the combustion chamber immediately prior to and during combustion. Turbulence within the chamber causes the uniform flame front within the combustion chamber to distort, creating a more convective mode of heat transfer and sending "tongues" of highly reactive radicals from the flame front into the unburned mixture. These conditions combine to promote more rapid combustion of the unburned mixture than would occur in a quiescent, low-turbulence mixture.
While some turbulence is created within the combustion chamber by the mere act of the piston compressing the air-fuel mixture, the effect can be heightened by modifying the shape of the combustion chamber so that the motion of the piston interacts with the chamber contours to cause greater turbulence during compression. It is especially desirable if the greatest turbulence occurs in the end gas near the end of the compression stroke, just before ignition. Examples of chamber configurations designed to promote mixing and/or turbulence are found in U.S. Patents 4,838,222, 4,844,040, 5,115,774, and 5,115,776.
It is further desirable for the combustion period to be approximately constant, when measured in crankshaft degrees, over the range of speeds at which the engine will operate. This insures that regardless of engine speed, the expanding combustion gases will exert their peak pressures when the piston is at approximately the same position. A constant combustion period further insures that as the engine speed increases, thereby decreasing the compression time and causing more rapid temperature and pressure increases, the time for autoignition to occur will be proportionately decreased. A near-constant combustion period can be accomplished if the combustion chamber can be designed so that turbulence increases as engine speed is increased.
Autoignition is present in almost all internal combustion engines to some extent, and is difficult to totally eliminate. The goal of high-performance engine design is to minimize autoignition to such an extent that it no longer harms the engine's performance or its structure, while at the same time obtaining the highest compression ratio possible.